kilgota@xxxxxxxxxxxxxxxxxxxxxx wrote:
On Fri, 16 Jan 2009, Olivier Lorin wrote:
<snip>
Post gamma or white balance correction seemed to be more part of the
webcam
capabilities than the image state so that the new API to get these
features
is quite logical.
Hmmm.
While one is in the process of planning ahead, it it good to think of
everything. You have not yet seen any code for the sq905c cameras in
webcam mode, yet. Well, actually, you can if you want to. The needed
sequence of operations is used in the support for
"gphoto2 --capture-preview"
in libgphoto2/camlibs/digigr8/library.c.
What happens with this camera is, the frame data has been compressed and
therefore it is needed to know how much data is present for a frame
before downloading the rest of the frame. The length of the frame header
is set at 0x50 bytes, so one downloads that first. Here is a sample of
one such frame header:
0000 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0010 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0020 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0030 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0040 e0 5a 00 00 3b 3e 50 3b-36 00 ff 00 ff 00 ff 00 .Z..;>P;6.......
Now, most of this is just filler. The data size for the frame is to be
found in bytes 0x40 and 0x41 and it says that 0x5ae0 bytes must be
downloaded for this frame. I have never seen the next two bytes with
anything in them, so my reasonable conjecture is that they are similarly
used, as part of the data size field. It is just that in practice one
never sees a frame with so much data in it.
Now, my comment here is directed toward bytes 0x44 through 0x48. I do
not claim to know exactly what they mean. However, I would strongly
suspect that they have something to do with contrast, color balance,
intensity, and so on. Really, logic would seem to indicate that there is
no other point to providing those entries, which certainly do vary from
one frame to another and the readings in them do seem in some vague
sense to have something to do with the local light conditions, and sucn.
Therefore, confronted with my experience with these cameras, I would
tend to suspect that
"Post gamma or white balance correction seemed to be more part of the
webcam capabilities than the image state"
may not always be the case. For, here, with this camera, it appears that
some kind of related information is being passed with each individual
frame.
Yes, we've seen that more often. In this case the solution is simple, since we
need to do decompression in userspace anyways, just pass the entire frame
including the header (with the potential interesting bytes) to userspace in the
frame buffer, then userspace can get that info if it needs to.
A bigger problem is when this happens and the cam generates a standard format
like jpeg (with an additional header), but we haven't encountered cams like
this yet, so lets not worry about that for now.
Regards,
Hans
'
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